U.S. Pat. Nos. 3,475,345; 3,324,188; and 3,190,939 teach the use of large port hydrogen exchanged mordenite as a catalyst for hydrocarbon isomerization. The first named patent discloses that strong, hot acid leaching followed by mild acid leaching, finally followed by ammonium exchange results in an improved isomerization catalyst as compared to mordenite in which one or more of the 3 steps of the treatment is omitted.